1. Field of the Invention
The present invention relates to a stocking system, and more particularly, to a stocker driving system for effectively handling malfunctions that hamper operation of the stocking system and for coping with sudden increases in demand.
2. Description of the Related Art
Liquid crystal display devices (LCD) have gained in popularity. Having excellent picture quality, lower power consumption, thin profile, and low weight, LCDs have supplanted cathode ray tubes (CRT) for information display purposes. Recent developments in the semiconductor industry have enabled improvements, which have increased demand for LCDs.
LCDs are manufactured in a multi-step process, which includes fabricating an upper substrate and a lower substrate; coating and rubbing an alignment layer on the substrates for aligning a liquid crystal; attaching the upper substrate and the lower substrate; injecting the liquid crystal between the attached upper and lower substrates; and sealing the attached upper and lower substrates. After the alignment layer is coated and rubbed on the substrates, the substrates are attached using a sealant, and liquid crystal material is injected between the substrates through a sealant inlet-hole.
As LCDs are manufactured, it is often necessary to temporarily store partially completed LCDs as they go from one fabrication step to the next. Toward this end, partially completed LCDs are generally stored in a stocking system (hereinafter “stocker”), where they are stored and then transferred using a cassette transfer vehicle (CTV) from the stocker for the next process.
CTVs are classified into rail guided vehicle (RGV), which move along a rail, and automatic guided vehicles (AGV), which move along a designated path using automatic control systems. Recent advances in AGVs include laser guided vehicles (LGV), which are equipped with laser navigation systems has been developed and used.
FIG. 1 is a view illustrating a stocker according to the related art, in which a cassette is transferred through an AGV input/output port of a cassette stocker. A stocker includes a plurality of shelves or racks in which cassettes are stored, and input and output ports through which AGVs insert and retrieve cassettes. A cassette refers to a rack that holds one or more partially completed LCDs. An AGV transports LCDs in cassettes.
Referring to FIG. 1, a cassette containing partially completed LCDs is stored in a stocker 100 after a given fabrication step. At the appropriate time, AGVs retrieve and transfer the cassettes to the next manufacturing step. Based on the arrangement of the LCD manufacture equipment, AGV lines 101a and 101b may be installed at both sides of the stocker 100, on which the AGV may move to transport cassettes.
As illustrated in FIG. 1, stocker 100 includes AGV input ports 103a and 105a, AGV output ports 103b and 105b, corresponding to the AGV lines 101a and 101b. 
Accordingly, an AGV moving along the AGV line 101a loads and unloads cassettes respectively through input port 103a and output port 103b. Also, another AGV may move along AGV line 101b and load and unload cassettes respectively through input port 105a and output port 105b. 
When an AGV loads cassettes into input port 103a on one side region in the stocker 100, the cassettes are moved to the AGV output port 105b of the other side region.
However, a related art stocker 100 controlled to input/output the cassettes as described above has the following drawbacks. First, a pair of AGV input/output ports is installed at one side of the stocker corresponding to the AGV line. When many cassettes are suddenly input/output in the LCD manufacture process, the pair of AGV input/output ports cannot appropriately process the sudden increase in cassettes, resulting in a bottleneck. Second, if an AGV input/output port is disabled due to a malfunction, the entire stocker 100 is shut down.